Dec. 19, 2001 — Why do volcanoes erupt? There are actually two different kinds of volcanic eruptions: the ones that peacefully go “blurp,” and the ones that explosively go “blam.” The difference has to do with the composition of molten rock miles beneath the surface.

First, an explanation of the quiet kind of eruption:

Starting about 20 miles below the surface of the earth, in the 1,700-mile-thick layer called the mantle, conditions are hot enough to get rocks moving. Slowly, to be sure — in most places, less than an inch per year. But the rocks down there are definitely moving around.

The rock in the mantle layer is still solid. Immense pressure from the weight of all the rock overhead keeps everything from melting. It’s the same way pressure keeps hot water from boiling and turning into steam.

But it’s hot down there, ranging between 2,282 and 2,462 degrees Fahrenheit (1,250 to 1,350 degrees Celsius). Sometimes, slightly hotter chunks of solid rock start to move up toward the surface. As they rise, with less and less rock overhead, the pressure decreases. When the pressure finally gets low enough, the rocks can finally melt. Molten rock underground is called magma; once it reaches the surface it is known as lava.

This molten magma — semi-liquid, semi-solid — is less dense than the solid rock around it, and it starts to float to the surface, like oil in water. There aren’t any visible cracks between the rocks down in the mantle, like the ones we see at the surface. Instead, the magma rises up along the microscopic edges of the grains of minerals in the solid mantle rock.

When it gets into the crust layer, the top layer of the solid earth that’s about 20 miles thick, the magma finds cracks that let it ooze up to the surface. When the lava spurts and blurps out, it forms what are called shield volcanoes, for their gentle shield-like shape. Undersea volcanoes, which make up about 80 percent of the volcanoes on the earth, are shield volcanoes. So are Kilauea and Mauna Loa in Hawaii, and the volcanoes in Iceland.

Two kinds of crustBut there’s another way that magma forms, and the difference explains why some volcanoes ooze and some explode.

There are two kinds of crust on the surface of the earth. Continental crust is the “land” that we live on, and oceanic crust is at the bottom of the sea. The crust is broken into huge sections, called plates, that are moving around. In many places, plates of oceanic crust move toward the continental crust. Where they collide, the oceanic plate dives under the continental crust, because the continental crust is made up of lighter rocks.

Oceanic crust isn’t just rock. It’s got water in it, embedded in the molecules of the rock. Asbestos, for example, is a mineral that is as much as 10 percent water by weight.

Down in the heat and pressure of the mantle, the water is released. It’s hot, but under pressure — so it stays liquid. It rises up into the mantle rock above, lowering the melting temperature of that rock. That changes the solid rock to molten magma. Now, molten and less dense than the solid rock around it, the magma starts to rise.

But unlike the magma in the oozing kind of volcanoes, this magma carries molecules of water with it. At first the water molecules don’t turn to vapor because of the pressure they’re still under. But when the magma/water mix finally gets up into the crust, near the surface, the pressure is reduced. That lets the water evaporate into steam bubbles — like bubbles of gas in a soda bottle that you don’t see until you relieve the pressure by taking off the top.

As more and more gas is released from the magma, the pressure underground builds up, like shaking a soda bottle with the top still on. Finally the pressure underground is greater than the rocks can hold back, and wham-o! You have a violent eruption with those huge clouds of ash and flying chunks of rock. That’s called a stratovolcano.

Mexico’s Popocatepetl and the Philippines’ Pinatubo — as well as Mount St. Helens and Mount Rainier in Washington state — are all stratovolcanoes. So are any other volcanoes formed where water-bearing oceanic crust is diving under continental crust, in a process the geo folks call subduction.

Volcanic facts
Why don’t stratovolcanoes glow the way those rivers of lava do in Hawaii? The rocks and ash are plenty hot, between 1,800 and 2,000 degrees Fahrenheit. But since they mix with water and air as they fly into the sky, the end result is cooler than the red rivers of lava that pour out of non-explosive volcanoes. Those flows glow because they’re even hotter, around 2,500 degrees Fahrenheit.

Some additional volcanic facts:

There are about 200 volcanoes erupting on the earth every day — most of those underwater.

The U.S. Geological Survey says there are 67 active or potentially active volcanoes in the United States: seven in Hawaii, 16 in mainland Alaska, 24 in the Aleutian Islands, five each in Washington and Oregon, one each in Arizona and New Mexico, and two in Idaho — including Yellowstone National Park, which also takes in parts of Montana and Wyoming.

After the magma cools into rock, it erodes into little grains over many years, and is washed by rain and streams and rivers down into the sea. It settles onto the sea floor, and over time gets compressed into rock and becomes part of the oceanic crust.

Eventually it dives back down under the lighter continental crust into the mantle again, where it melts into magma again, comes back up again, and goes through the whole process over and over.

It takes millions of years for all this to happen, but scientists believe that all the oceanic crust rock on Earth has gone through that conveyor belt process at least 50 times since the earth formed 4.5 billion years ago.

David Ropeik is a longtime science journalist and currently serves at Director of Risk Communication at the Harvard Center for Risk Analysis. This article is drawn from the archives of “How and Why,” Ropeik’s column about scientific puzzlers.